Air conditioner evaporator coil keeps icing up

My air conditioner evaporator coil keeps icing up. Itís in an air-handler inside my house and it is an A-frame coil. I cleaned the filters and the coils donít seem to be dirty as far as I can tell airflow is fine. When it is iced up the ice covers the entire coils top to bottom on both sides. But when the ice is off and the unit is running the frost line is only half way up the coils on both sides. The top of the coils doesnít seem to be refrigerating at all (not wet or cool). This would indicate to me that its low on gas but why is it that when it is iced up the entire coils is iced. I am an appliance repairman and I have the equipment to make all necessary repairs.

I havenít put gauges on it to see the pressure yet just wanted to see if anyone had any thoughts on this and any help would appreciated.

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Because it takes X amount of time for it to freeze/ice up the lower portion where you see the frost.
Once that portion becomes ice.
The liquid refrigerant can travel further up the coil and freeze it also.